1. Field of the Invention
The present invention generally relates to a resin molding technique, and more particularly, to a resin molding condition control method for controlling temperature conditions for metal molds and pressurizing conditions for the resin during resin molding. The intention is particularly applied to molded items requiring high accuracy such as plastic lenses and the like.
2. Description of Related Art
In recent years, with the spread of appliances having optical systems such as a projection type television receiver, video camera, CD player or the like, mass produced plastic lenses meet the rapidly increasing demand for these optical systems have come into actual use for wide applications and provide a cost advantage, a light weight and high aberration correcting capacity by an aspherical structure, etc.
In these plastic lenses, a particularly high accuracy in the configuration is required as compared with resin molded articles in general, and as one means for realizing such a high accuracy, there has conventionally been proposed a method for raising the temperature at the resin surface layer after the resin has been cooled down to a certain extent, for example, in Japanese Patent Publication Tokkosho No. 61-19920 or Tokkohei No. 1-34132.
In the known method described above, by making uniform, the non-uniform density distribution resulting from large differences in cooling/hardening speeds of resin from place to place, through softening on the surface layer of the resin and rearrangement of the resin by melting, uneven shrinkage or contraction (so-called shrinkage or sink mark) may be reduced to achieve high accuracy in the configuration.
According to research by the present inventors, in order to fully to obtain the effect by the temperature raising, it is necessary that the resin be almost completed in its hardening before starting the temperature raising, and also upon completion of the temperature raising, the resin be softened and melted to such an extent that the non-uniformity of the pressure distribution may be eliminated.
The temperature which satisfies the above conditions may be determined by the temperature dependency of viscoelastic characteristics of the resin to be employed, and although there may be some differences depending on resins to be dealt with, it is estimated that the above temperature is a temperature approximately equal to the glass transition temperature for the former condition, and a temperature higher than the glass transition temperature by about 10.degree. to 60.degree. degrees C. with respect to the latter condition.
It is noted, however, that the glass transition temperature has a pressure dependency, and becomes higher as the pressure is raised.
Meanwhile, the completion of the transfer of the resin configuration is at a time point when the resin has finished its phase change from a liquid phase to a solid phase, i.e., at a time point when the resin has reached the glass transition temperature, and it is considered that a ratio of a specific volume at this time (glass transition specific volume) to a specific volume under normal temperature and normal pressure, may be concluded as the molding shrinkage factor. However, it should be noted that the resin has a large compressibility, and the glass transition specific volume and consequently, the molding shrinkage factor is varied to a large extent by an applied pressure.
Since the physical properties of resin (e.g., viscoelastic characteristics, specific volume) strongly depend on the pressure, the pressure condition control is important as well as the temperature condition control in the precision molding. Moreover due to the fact that they are not independent requirements, but mutually related, it becomes necessary to provide a molding condition control for coordinating the control of the temperature conditions and pressure conditions.
In the conventional techniques, however, there have been effected no coordinated and rational molding condition control from the viewpoints of both the temperature and pressure dependency in the physical properties of resin, especially, taking into consideration the mutual interrelationship between these properties.